Is it permissible to travel from Cairo to Alexandria (210 km) accompanied by my adult nephew (36 years old) and my daughter (14 years old), and does the presence of the mother with her daughter make a mahram unnecessary?

A woman is not permitted to travel without a mahram (definition), whether the journey is short or long, due to the Prophet’s (peace be upon him) saying: “A woman is not to travel except with a dhī mahram (a male whom she is perpetually forbidden to marry).” Your sister's son is not a mahram to your daughter, and your presence does not suffice for a mahram, because one of the conditions for a mahram is that he must be male. Ibn Uthaymeen and Ibn Baz were asked about this and both affirmed that it is impermissible for a woman to travel except with a mahram, and that a sister’s husband is not a mahram to his wife’s sister.